It is known in the art to use a lost motion hydraulic actuator for transmitting cam motion to a valve of an internal combustion engine, to vary the valve timing through electronic solenoid control of fluid exhaust from the actuator, and to provide hydraulic damping of the valve seating action through temporary restriction of hydraulic flow. The structure and operation of two such actuator embodiments are disclosed in detail in U.S. Pat. Nos. 5,158,048 Robnett et al. issued Oct. 27, 1992, and 5,216,988 Taxon issued Jun. 8, 1993, both assigned to the assignee of the present invention. The disclosures of both these patents are hereby incorporated by reference in this disclosure.
In these prior mechanisms, the damping device, comprising a sharp-edged orifice, is fully effective only after closing of a free flow fluid path, including communication ports which are closed by a valve driven piston upon closing motion of the engine valve to near the point of seating. In some instances, some hydraulic hammering noise has been observed in operation, apparently due to the rapid change in hydraulic flow rate that occurs during the change from free flow, when the communication ports are open, to damped flow, when these ports have been cut off. Improved arrangements to reduce the occurrence of hydraulic hammer shock and noise are desired.